1. Field of the Invention
The present invention relates to a data transmitting apparatus and, more particularly, to a transmitting apparatus which transmits variable-length code data.
2. Related Background Art
In a digital data transmitting apparatus, an error detection correction code, a sync block (SYNC), and identification data (ID) are added to data to be transmitted, the resultant data is divided into error correction blocks (or sync blocks) containing a predetermined data amount and is transmitted to a transmitting medium.
There is also a case where data to be transmitted is previously compressed and coded and is subsequently divided into sync blocks.
For example, in a conventional apparatus of a digital video tape recorder (VTR), image data to be recorded is compressed by a fixed-length coding method by a subsampling and/or a DPCM (Differential PCM) coding or the like, error detection correction codes C.sub.1 and C.sub.2 by product codes are added to the image data of one image plane, and the image data is divided into error correction blocks and recorded onto a magnetic tape.
FIG. 1 shows a fundamental structure of the error correction block. According to the fixed-length coding method, a compression ratio of 1/2 to 1/4 is obtained.
As compared with the fixed-length coding method, according to a variable-length coding method using a run length code or a Huffman code, a high compression ratio of about 1/10 to 1/50 is obtained, so that much attention has been paid to the variable-length coding method for use in transmission of a moving image.
There is, however, a possibility such that when variable-length coding data is sequentially inserted into the data portion of the error correction block, the integrated variable-length coding data will lie in two error correction blocks.
And in the variable-length coding method, there is a case where if the preceding variable-length code cannot correctly be decoded, errors are propagated. In such a case, an image quality is extremely deteriorated.